For a mobile communication system such as mobile telephones, the third-generation scheme has started providing service through the use of CDMA scheme. The 3GPP (3rd Generation Partnership Project) (R) has been discussed over the next-generation mobile communication system (LTE: Long Term Evolution) which makes communication at a higher speed possible (see Non-Patent Document 1 below). In the project, reduction of delay in transmission is being discussed in addition to enhancement in transmission rate.
In the event of initiation of communication between a base station unit (evolved Node B: eNB) and a mobile station unit (User Equipment: UE) serving as a wireless terminal prepares in a mobile communication system, a channel is prepared through which the UE first transmits data. The 3GPP calls this channel a random access channel (RACH) and calls a communication starting procedure using a RACH random access (see Non-Patent Document 2 below).
A RACH includes minimum information that causes the eNB to recognize transmission from the UE. A RACH is used at the initiation of the communication and the subsequent communication uses an individual channel (or common channel). A RACH can be shared by a number of UEs unless two or more UEs concurrently use the RACH. For the above, a RACH uses an identifier called a signature with which the eNB can identify UEs concurrently transmitting data through the RACH.
Random access is carried out in the following four cases of: (1) transmission of first data; (2) establishment uplink synchronization when downlink data arrive; (3) request of uplink data transmission when uplink data arrive; and (4) establishment of synchronization with a destination base station when handover occurs. The direction from the eNB to the UE is defined as “downlink (DL)”, and the reverse direction is defined as “uplink (UL)”.
Here, when (1) transmission of first data or when (3) transmission of uplink data, the UE randomly selects one signature from available signatures (preambles) and uses the selected signature (Contention Based Random Access Procedure). Accordingly, there is a possibility, however being low, of two or more UEs concurrently transmit data using the same signature.
Conversely, the eNB allocates a dedicated signature to the UE in advance when (2) transmission of downlink data. A possible collision of a signature may cause an instantaneous interruption of the connection or communication disconnection of the communication when (4) handover occurs. Therefore, a dedicated signature is allocated to the UE that is the subject of the handover (Non-contention Based Random Access Procedure).
(a) Contention Based Random Access Procedure:
FIG. 20 illustrates an example of a random access procedure used in the above cases (1) and (3) disclosed in the non-Patent Document 2.
Upon uplink data arrival, the UE transmits a message (Random Access Preamble) #1-1 (uplink transmission request) containing a signature randomly selected to the eNB through the RACH (step S101). At that time, there is a possibility of occurrence of a contention because two or more UEs concurrently start transmission through the use of the same signature. However, even when a contention occurs, the eNB cannot recognize the effective ID of each UE and cannot therefore grasp that the contention occurs between which UEs.
Upon receipt of the message #1-1 (the signature), the eNB reply with the response message (Random Access Response) #1-2 to the received message #1-1 (step S102) along with a timing advanced as synchronization signals for uplink communication, an uplink grant for transmission permission, and others. If a number of UEs 20 concurrently transmit requests through the RACH, the eNB 10 returns the response message #1-2 to the UEs 20.
Next, the UE, which receives the response message #1-2, transmits the ID of the UE itself via a message (Scheduled Transmission) #1-3 to request the eNB 10 to schedule UL communication to the eNB (step S103).
Upon receipt of the message #1-3, the eNB recognizes the effective ID of the UE (hereinafter also called a terminal ID) and thereby can grasp that the contention of the signature occurs between which UEs. If a contention occurs, the eNB transmits a message (Contention Resolution) #1-4 to the UEs in question to resolve the contention (step S104).
(b) Non-Contention Based Random Access Procedure
FIG. 21 illustrates an example of a random access procedure (Non-contention Based Random Access Procedure) used in the above cases (2) and (4) disclosed in the Non-Patent Document 2.
The eNB allocates a dedicated signature to each UE under the control of the eNB via a message (Random Access Preamble assignment) #2-1 in advance (step S20).
The UE issues UL synchronization request to the eNB using the dedicated signature allocated by the eNB via the message #2-1. In other words, the UE transmits a message #2-2 containing a dedicated signature to the eNB through the RACH (step S202).
Upon receipt of the message #2-2, the eNB replies with the response message #2-3 to the received message (step S203) along with a timing advanced as synchronization signal, an uplink grant for transmission permission, and others.    Non-Patent Document 1: 3GPP, “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN)”, TR25.913 V7.3.0, Release 7, March 2006    Non-Patent Document 2: 3GPP, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)”, TS36.300, Release 8, V8.1.0, June 2007
As described above, since the non-Patent Document 2 examines two kinds of procedure of random access, different procedures concurrently proceed in, for example, cases of (2) establishment uplink synchronization when downlink data arrive and (3) request of uplink data transmission when uplink data arrive.
Since different procedures concurrently proceeding as the above requires respective resources (such as signature), two kinds of signature are allocated while the procedures are proceeding so that the signatures are wasted.